1. Field of the Invention
This invention relates to transparent polycrystalline ceramic scintillators and methods of preparing the same. The invention also relates to gadolinium-yttrium oxide ceramic scintillators with europium added as an activator, which are optically transparent and whose cubic structures are maintained even when gadolinium oxide constitutes more than 50% of the scintillator. The scintillators of the present invention also have high light yield due to excellent luminescent characteristics in the visible light region.
2. Related Art
A scintillator is a material that emits visible light by absorbing radiation such as X-rays and gamma rays. Occasionally normal phosphor is used as a scintillator, but in most cases high density and high atomic number materials are used or added. Scintillators are used for various purposes, e.g., measuring the effects of radiation as a quarantine treatment, as industrial references, as medical care diagnostics, etc. However, the most typical application is in medical computerized tomography (CT-scanner). In a CT-scanner, the scintillator emits visible light by absorbing radiation, and a photon sensor, e.g., a scintillator-charge-coupled device (CCD), transforms the visible light into an electrical signal.
Generally, computerized tomography (CT)-scanners use single crystal CdWO4 as scintillators. Scintillators require high optical transmittance to enhance the photon transmission efficiency to the photon sensor.
Polycrystalline ceramic scintillators have been developed to overcome the shortcomings of single crystal scintillators and to acquire better and preferred physical properties. Existing polycrystalline ceramic scintillators fulfill these requirements, however it is difficult to achieve optically transparent conditions through conventional ceramic manufacturing processes. This difficulty in manufacturing conventional transparent ceramic scintillators is one of the most limiting factors of ceramic scintillators.
Among various ceramic scintillators, gadolinium-yttrium oxide ceramic scintillator, with europium added as an activator, is considered to be the most representative substance that overcomes these difficulties. This scintillator is based on yttrium oxide, which is known to be sinterable to transparent ceramic. A typical scintillator has 3% europium oxide and 30% gadolinium oxide substituted into the host material, yttrium oxide. Europium oxide functions as a luminescent activator, and gadolinium oxide functions to elevate the absorption coefficient for the radiation, e.g., X-rays, by substituting yttrium oxide. Substituting high density and high atomic number gadolinium oxide into yttrium oxide, and adding europium oxide as the activator, creates a scintillator that efficiently absorbs radiation.
The problem with conventional gadolinium-yttrium oxide is that the quantity of gadolinium oxide substituted is limited to less than 50% (Greskovich et al., American Ceramic Society Bulletin 71:1120-1130 (1992)). In scintillators with high gadolinium oxide content, the crystalline phase is changed from cubic to monoclinic. Monoclinic ceramics have low transmittance and low luminescent characteristic. Thus, to enhance the light yield, a need exists for a scintillator containing 50% or more gadolinium oxide that maintains its cubic crystal structure.